Limited efficacy of antimicrobial metaphylaxis in finishing pigs: A randomized clinical trial.

Pigs that die from pathogens associated with porcine respiratory disease complex (PRDC) in the late finishing period represent a significant economic wastage. While it is common to apply antimicrobial metaphylaxis (AM) to control PRDC, there are few studies exploring the potential cost-saving benefits of AM. In this study we examined the value of using AM in commercially reared, late finishing pigs, from farms with endemic PRDC. A total of 732 pigs from four AIAO wean to market sources, were blocked into 2 matching cohorts, based on enrollment body weight, sex, and rectal temperature. The cohorts received either control (C) or AM (Tulathromycin 2.5mg/kg IM, Zoetis, Florham Park, NJ, USA). Post treatment weight gain over the 21 day period was used as a measure of health and productivity. The AM treated pigs in the lowest weight quartile at enrollment, showed a significantly improved weight gain over controls (18.5 kg vs. 16.4 kg, mean difference=2.1 kg, CI 1.10-3.10, p=0.005) that was not evident in any other starting weight quartiles. These results indicate that the biological advantage and associated improvement in growth efficiency associated with the use of AM against PRDC, is only conferred to a specific sub-set of animals. The economic advantage of this strategy is therefore, only likely if the indicators of potential benefit (e.g., lighter weight cohort) can be reliably established. Further studies are needed to determine whether targeted AM could be effectively applied across the industry.